This invention relates to a apparatus for molding a bonded product, made of sawmill wood waste or other suitable fibrous waste matter, having sufficient continuous thickness, strength and other characteristics to enable the product to be utilized satisfactorily as a lumber substitute. The invention enables the product to be manufactured at a high production rate despite its substantial continuous thickness, without inordinate financial investment in equipment.
The lumber industry is presently confronted with the severe problem of disposing of vast amounts of bark and other wood waste accumulating at sawmills in the lumber-producing regions of the United States. Although limited amounts of the waste may be used in paper production or as fuel, there remain huge quantities of bark, sawdust and shavings for which no adequate market presently exists. In the past, disposal of the material by burning caused sufficient air pollution that the practice has now been almost completely prohibited. More recent types of disposal result in land pollution or land disfiguration, which is equally undesirable. Industries producing other types of fibrous vegetable waste, such as bagasse from sugar cane processing, are faced with a similar disposal problem.
In recent years the wood products industry has introduced an assortment of resin-bonded particle board products to the market in an attempt to develop a demand for sawmill wastes. Several of these products, and the processes by which they are made, are exemplified in Roman U.S. Pat. No. 2,446,304, Goss U.S. Pat. No. 2,581,652, Elmendorf et al, U.S. Pat. No 2,381,269, and Schueler U.S. Pat. No. 3,309,444. A significant and almost universal characteristic of the various particle board products is that they are prepared by compressing and curing them over an extended period of time in a relatively expensive hot platen press. This occupies the press and thus delays production for the period of time necessary to insure sufficient curing of the resin binder the length of curing time being a function primarily of the thickness of the compressed material. (The temperature of the press cannot be raised significantly to shorten the curing time because charring or scorching of the material might then result.) The above-mentioned Goss patent has suggested that the delay problem might be alleviated by removing the pad of compressed material from the press without curing it and then transfering it to a separate oven. This proposal is unacceptable, however, because relieving the external pressure prior to curing will yield an inadequately bonded product. Consequently, to avoid tying up expensive platen presses for an inordinate period of time and thereby delaying production unduly while waiting for the material to cure, particle board manufacturers have generally limited the maximum final thickness of the product to between 1/2 and 3/4 inch. Such limited thickness requires only a fraction of an hour for curing in a platen press under normal temperature conditions as opposed, for example, to at least two hours for material of 11/2 inch thickness.
Unfortunately the practical limitations on the maximum thickness of particle board, imposed by the shortcomings of present production methods, has severely limited the market for which particle board is suitable. The product is satisfactory for use as a decorative or protective siding, but lacks the continuous solid thickness necessary to permit its being cut to standard dimension lumber sizes, for example the 11/2 inch thickness now required of a nominal 2 .times. 4 stud. This limitation has, up to now, effectively foreclosed particle board from entry into the very substantial market for framing lumber, i.e. the dimension lumber market, wherein the demand for the product might well multiply many times and thereby offer some solution to the present sawmill waste problem.
An additional drawback of present particle board for this application is that it is generally denser than lumber, tending to require relatively greater shipping costs which would place particle board at a competitive disadvantage in the dimension lumber market. Moreover there has apparently been a failure in the industry to discriminate between those types of bark which may be used in particle board without adversely affecting its strength, and those types which detract from its strength. This has generally resulted in a reluctance to use any kind of bark in a particle board product where strength is a factor. These problems have tended further to limit the effectiveness of particle board as an answer to the overall wood waste disposal problem.
Accordingly a great need presently exists for a particle board product which utilizes a maximum amount of vegetable waste matter in its manufacture and has thickness and other characteristics sufficient to enable such product to be sold competetively in the dimension lumber market as a lumber substitute, particularly for use as studding, thereby establishing a requirement for a much greater percentage of sawmill and other fibrous vegetable waste matter than is presently being utilized. In order to accomplish this purpose a new apparatus is required by which much thicker continuous layers of the particle board material may be cured under pressure than has been accomplished in the past, such apparatus being capable of a high production rate despite the need for long curing times and yet not requiring an unusually high investment in equipment to achieve such production rate.